Switch valve for an extracorporeal blood circuit and circuit including such a switch valve

ABSTRACT

A switch valve for use in an extracorporeal blood flow circuit comprises a valve housing having a chamber, four openings communicating with the chamber, and a valve member located in the valve chamber. A first opening is to be connected to a patient via an arterial cannula, a second opening is to be connected to a patient via an venous cannula, a third opening is to be connected to a first inlet/oulet of a blood treatment device, and a fourth opening is to be connected to a second inlet/outlet of a blood treatment device. The valve member is movable within the valve housing to fluidly connect the first opening to either the third or the fourth opening and to fluidly connect the second opening to either the third or the fourth opening. The width of the valve member is smaller than a peripheral dimension of the openings.

[0001] The present patent application is a divisional application ofU.S. patent application Ser. No. 09/425,124 filed on Oct. 22, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the invention

[0003] The present invention relates to a method and device formeasuring blood flow rate in a blood access. Blood is taken out from thebody of a mammal to an extracorporeal blood circuit through a bloodaccess, via needles or a catheter.

[0004] 2. Description of the Related Art

[0005] There are several types of treatments in which blood is taken outin an extracorporeal blood circuit. Such treatments involve, forexample, hemodialysis, hemofiltration, hemodiafiltration,plasmapheresis, blood component separation, blood oxygenation, etc.Normally, blood is removed from a blood vessel at an access site andreturned to the same blood vessel or at another location in the body.

[0006] In hemodialysis and similar treatments, an access site iscommonly surgically created in the nature of a fistula. Blood needlesare inserted in the area of the fistula. Blood is taken out from thefistula via an arterial needle and blood is returned to the fistula viaa venous needle.

[0007] A common method of generating a permanent access site havingcapability of providing a high blood flow and being operative duringseveral years and even tens of years, is the provision of anarterio-venous fistula. It is produced by operatively connecting theradial artery to the cephalic vein at the level of the forearm. Thevenous limb of the fistula thickens during the course of several months,permitting repeated insertion of dialysis needles.

[0008] An alternative to the arterio-venous fistula is thearterio-venous graft, in which a connection is generated from, forexample, the radial artery at the wrist to the basilic vein. Theconnection is made with a tube graft made from autogenous saphenous veinor from polytetrafluorethylene (PTFE, Teflon). The needles are insertedin the graft.

[0009] A third method for blood access is to use a silicon, dual-lumencatheter surgically implanted into one of the large veins.

[0010] Further methods find use in specific situations, like a no-needlearterio-venous graft consisting of a T-tube linked to a standard PTFEgraft. The T-tube is implanted in the skin. Vascular access is obtainedeither by unscrewing a plastic plug or by puncturing a septum of saidT-tube with a needle. Other methods are also known.

[0011] During hemodialysis, it is desirable to obtain a constant bloodflow rate of 150-500 ml/min or even higher, and the access site must beprepared for delivering such flow rates. The blood flow in an AV fistulais often 800 ml/min or larger, permitting delivery of a blood flow ratein the desired range.

[0012] In the absence of a sufficient forward blood flow, theextracorporeal circuit blood pump will take up some of the alreadytreated blood entering the fistula via the venous needle, so calledaccess or fistula recirculation, leading to poor treatment results.

[0013] The most common cause of poor flow with AV fistulas is partialobstruction of the venous limb due to fibrosis secondary to multiplevenipunctures. Moreover, stenosis causes a reduction of access flow.

[0014] When there is a problem with access flow, it has been found thataccess flow rate often exhibit a long plateau time period with reducedbut sufficient access flow, followed by a short period of a few weekswith markedly reduced access flow leading to recirculation andultimately access failure. By constantly monitoring the evolution of theaccess flow during consecutive treatment sessions, it is possible todetect imminent access flow problems.

[0015] Several methods have been suggested for monitoring recirculationand access flow. Many of these methods involve injection of a markersubstance in blood, and the resultant recirculation is detected. Themethods normally involve measurement of a property in the extracorporealblood circuit. Examples of such methods can be found in U.S. Pat. No.5,685,989, U.S. Pat. No. 5,595,182, U.S. Pat. No. 5,453,576, U.S. Pat.No. 5,510,716, U.S. Pat. No. 5,510,717, U.S. Pat. No. 5,312,550, etc.

[0016] Such methods have the disadvantage that they cannot detect whenthe access flow has decreased to such an extent that recirculation is atrisk, but only when recirculation prevails. Moreover, it is a drawbackthat injection of a substance is necessary.

[0017] A noninvasive technique that allows imaging of flow through AVgrafts is color Doppler ultrasound. However, this technique requiresexpensive equipment.

[0018] The measurement of access flow rate necessitates the reversal ofthe flows in the extracorporeal circuit. A valve for such reversal isshown in i.a. U.S. Pat. No. 5,605,630 and U.S. Pat. No. 5,894,011.However, these valve constructions comprise dead ends in which blood maystand still for a long time and coagulate, which is a drawback.

BRIEF SUMMARY OF INVENTION

[0019] An object of the present invention is to provide a method and adevice for measuring the access flow rate without interfering with theblood and without injecting a substance in blood.

[0020] Another object of the invention is to provide a method and adevice for measuring access flow rate without measuring on the blood inthe extracorporeal blood circuit or in the access or blood vessel.

[0021] According to the invention, it is required to reverse the bloodflow through the access. Thus, a further object of the invention is toprovide a valve for reversing the blood flow.

[0022] A still further object of the invention is to provide a methodfor determining when the blood flow rate is so small that risk forrecirculation prevails.

[0023] These objects are achieved with a method and an apparatus forestimating fluid flow rate (Qa) in a fluid flow access, comprisingremoving a first fluid flow from said access at a removal position to anexternal flow circuit comprising a dialyzer having a semipermeablemembrane, said first fluid flow passing along said membrane at one sidethereof and a dialysis fluid being emitted from the other side thereof,and returning said first fluid flow from said external flow circuit tosaid access at a return position downstream of said removal position,measuring a first variable which is essentially proportional to aconcentration (Cd norm) of a substance in said dialysis fluid emittedfrom the dialyzer, reversing the removal position with the returnposition and measuring a second variable which is essentiallyproportional to the concentration (Cd rev) of said substance in saiddialysis fluid in the reversed position; and calculating the fluid flowrate (Qa) in said flow access from said measured concentrations.

[0024] Preferably, the calculation of the fluid flow rate in said flowaccess takes place by calculating the ratio between the first and thesecond variable and using the formula: Cd norm/Cd rev=1+K/Qa, in whichCd norm and Cd rev are values proportional to the concentrations of saidsubstance in the dialysis fluid in the normal and reversed positions,respectively, and K is the clearance of the dialyzer and Qa is theaccess flow rate.

[0025] The blood flow access may be in a mammal for obtaining access toa blood vessel, such as a hemodialysis access in the nature of anarterio-venous shunt or fistula. In the latter case, the dialyzerclearance K is replaced by the effective dialyzer clearance Keffobtained by taking into account a cardiopulmonary recirculation and inthe normal position.

[0026] The substance is preferably selected from the group of: urea,creatinine, vitamin B12, beta-two-microglobuline and glucose, or may bean ion selected from the group of: Na⁺, Cl⁻, K⁺, Mg⁺⁺, Ca⁺⁺, HCO₃ ⁻,acetate ion, or any combination thereof as measured by conductivity; andwherein said concentration is measured as the concentration differencebetween the outlet and the inlet of the dialyzer, if applicable.

[0027] It is possible to measure the actual concentration of thesubstance. However, since only the ratio between the concentrations inthe normal and the reversed position, respectively, is needed, it ispossible to measure a value, which is proportional to the concentrationof said substance, whereby said value is used in place of saidconcentration. Said property may be the blood concentration of saidsubstance in the external circuit, either before or after the dialyzer.Alternatively, the relative whole body efficiency (Kwh/V) may be used,as explained in more detail below.

[0028] The effective clearance Keff may be obtained by the equationKeff=Qd*Cd/Cs, where Qd is the flow of dialysis fluid emitted from thedialyzer, Cd is the concentration of said substance in said dialysisfluid and Cs is the concentration of said substance in systemic venousblood.

[0029] A method of measuring the concentration (Cs) of said substance insystemic venous blood comprises the steps of: stopping the blood flow inthe external flow circuit for a time period sufficient to allow thecardiopulmonary circulation to equalize; starting the blood flow in theexternal flow circuit with a slow speed to fill the arterial line withfresh blood before the measurement; and measuring the equalizedconcentration of said substance in the dialysis fluid at a low dialysateflow rate or at isolated ultrafiltration. It is advantageous to make themeasurement of the effective clearance at the initiation of thetreatment.

[0030] The concentration (Cs) of said substance in systemic venous bloodmay be estimated by: calculating a whole body mass of urea (Murea) inthe body of the patient, estimating or measuring the distribution volume(V) of urea in the body of the patient; and estimating the concentration(Cs) of said substance in the blood by dividing the whole body mass ofurea with the distribution volume. In this way, the mean concentrationof urea in the whole body is obtained. However, the mean concentrationin the whole body is slightly higher than the urea concentration in thesystemic blood, except at the start of the treatment. Thus, thiscalculation should preferably be done or be extrapolated to the start ofthe treatment.

[0031] It is possible to discriminate between the condition when accessor fistula recirculation has developed and not. A method for thatpurpose would be: changing the blood flow rate (Qb); monitoring theconcentration of said substance in the dialysate emitted from thedialyzer; and detecting a possible fistula recirculation in the normalposition by correlating a change in said concentration to said change ofthe blood flow rate.

[0032] Preferably, the blood flow rate is decreased and a correspondingdecrease in the urea concentration is monitored, and the absence of sucha decrease being indicative of fistula recirculation.

BRIEF DESCRIPTION OF DRAWINGS

[0033] Further objects, advantages and features of the invention appearsfrom the following detailed description of the invention with referenceto specific embodiments of the invention shown on the drawings, in which

[0034]FIG. 1 is a partially schematic view of a forearm of a patientprovided with an AV fistula;

[0035]FIG. 2 is a schematic diagram of an extracorporeal dialysiscircuit;

[0036]FIG. 3 is a schematic diagram of the blood flow circuit in apatient and in the attached extracorporeal blood circuit;

[0037]FIG. 4 is a schematic diagram similar to FIG. 3, but with theextracorporeal circuit in an alternative reversed position;

[0038]FIG. 5 is a schematic diagram of a blood flow circuit including aswitch valve;

[0039]FIG. 6 is a diagram of the dialysis fluid urea concentrationversus time, including a portion with reversed flow access according tothe invention;

[0040]FIG. 7 is a schematic diagram similar to the diagram of FIG. 5comprising an alternative valve arrangement;

[0041]FIG. 8 is schematic diagram similar to the diagram of FIG. 7showing the valve arrangement in an idle position;

[0042]FIG. 9 is schematic diagram similar to the diagram of FIG. 7showing the valve arrangement in a reversed position;

[0043]FIG. 10 is a schematic diagram similar to FIG. 5 with the pump inan alternative position;

[0044]FIG. 11 is a diagram showing calculations with relative whole bodyefficiency;

[0045]FIG. 12 is a cross-sectional view of a valve housing to be used inthe schematic diagram of FIGS. 5 and 7 to 10;

[0046]FIG. 13 is a bottom view of a valve member intended to be insertedin the valve housing of FIG. 12; and

[0047]FIG. 14 is a partially schematic plan view of the valve housing ofFIG. 12.

DETAILED DESCRIPTION OF INVENTION

[0048] For the purpose of this description, an access site is a site inwhich a fluid in a tube can be accessed and removed from and/or returnedto the tube. The tube may be a blood vessel of a mammal, or any othertube in which a fluid is flowing. The access flow rate is the flow rateof the fluid in the tube or blood vessel immediately upstream of theaccess site or removal position.

[0049]FIG. 1 discloses a forearm 1 of a human patient. The forearm 1comprises an artery 2, in this case the radial artery, and a vein 3, inthis case the cephalic vein. Openings are surgically created in theartery 2 and the vein 3 and the openings are connected to form a fistula4, in which the arterial blood flow is cross-circuited to the vein. Dueto the fistula, the blood flow through the artery and vein is increasedand the vein forms a thickened area downstream of the connectingopenings. When the fistula has matured after a few months, the vein isthicker and may be punctured repeatedly. Normally, the thickened veinarea is called a fistula.

[0050] An arterial needle 5 is placed in the fistula, in the enlargedvein close to the connected openings and a venous needle 6 is placeddownstream of the arterial needle, normally at least five centimetersdownstream thereof.

[0051] The needles 5 and 6 are connected to a tube system 7, shown inFIG. 2, forming an extracorporeal circuit comprising a blood pump 8,such as a dialysis circuit. The blood pump propels blood from the bloodvessel, through the arterial needle, the extracorporeal circuit, thevenous needle and back into the blood vessel.

[0052] The extracorporeal blood circuit 7 shown in FIG. 2 furthercomprises an arterial clamp 9 and a venous clamp 10 for isolating thepatient from the extracorporeal circuit should an error occur.

[0053] Downstream of pump 8 is a dialyzer 11, comprising a bloodcompartment 12 and a dialysis fluid compartment 13 separated by asemipermeable membrane 14. Further downstream of the dialyzer is a dripchamber 15, separating air from the blood therein.

[0054] Blood passes from the arterial needle past the arterial clamp 9to the blood pump 8. The blood pump drives the blood through thedialyzer 11 and further via the drip chamber 15 and past the venousclamp 10 back to the patient via the venous needle. The drip chamber maycomprise an air detector, adapted to trigger an alarm should the bloodemitted from the drip chamber comprise air or air bubbles. The bloodcircuit may comprise further components, such as pressure sensors etc.

[0055] The dialysis fluid compartment 13 of the dialyzer 11 is providedwith dialysis fluid via a first pump 16, which obtains dialysis fluidfrom a source of pure water, normally RO-water, and one or severalconcentrates of ions, metering pumps 17 and 18 being shown for meteringsuch concentrates. The preparation of dialysis fluid is conventional andis not further described here.

[0056] An exchange of substances between the blood and the dialysisfluid takes place in the dialyzer through the semipermeable membrane.Notably, urea is passed from the blood, through the semipermeablemembrane and to the dialysis fluid present at the other side of themembrane. The exchange may take place by diffusion under the influenceof a concentration gradient, so called hemodialysis, and/or byconvection due to a flow of liquid from the blood to the dialysis fluid,so called ultrafiltration, which is an important feature ofhemodiafiltration or hemofiltration.

[0057] From the dialysis fluid compartment 13 of the dialyzer is emitteda fluid called the dialysate, which is driven by a second pump 19 via aurea monitor 20 to drain. The urea monitor continuously measures theurea concentration in the dialysate emitted from the dialyzer, toprovide a dialysate urea concentration curve during a dialysistreatment. Such urea concentration curve may be used for severalpurposes, such as obtaining a total body urea mass, as described in WO9855166, and to obtain a prediction of the whole body dialysis dose Kt/Vas also described in said application. The content of WO 9855166 isincorporated in the present specification by reference.

[0058] As described above, the present invention provides a method ofnon-invasively measuring the access flow in the fistula immediatelybefore the arterial needle, using the urea monitor and the dialysiscircuit as shown in FIG. 2.

[0059] By measuring the dialysis urea concentration during normaldialysis and then reversing the positions of the needles and measuringthe dialysis urea concentration with the needles in the reversedposition, it is possible to calculate the blood flow in the bloodaccess, without the addition of any substance to blood or the dialysisfluid.

[0060]FIG. 3 shows a simplified schematic diagram of the blood vesselcircuit of a patient and a portion of the dialysis circuit according toFIG. 2. The patient blood circuit comprises the heart, where the rightchamber of the heart is symbolized by an upper pump 21 and the leftchamber of the heart is symbolized by a lower pump 22. The lungs 23 arelocated between the upper and lower pump. From the outlet of the leftchamber pump 22 of the heart, the blood flow divides into a first branch24 leading to the access 25, normally in the left forearm of thepatient, and a second branch 26 leading to the rest of the body, such asorgans, other limbs, head, etc. symbolized by a block 27. Bloodreturning from the body from the organs etc., i.e. from block 27,combines with blood returning from the access and enters the rightchamber pump 21.

[0061] The cardiac output flow rate is defined as Qco and the flow rateof the access is defined as Qa, which means that Qco-Qa enters the block27. The venous blood returning from block 27 before being mixed withblood from the access, the systemic venous blood, has a ureaconcentration of Cs. The blood leaving the left chamber pump 22 has aurea concentration of Ca equal to that passing out to the access 25 aswell as to the block 27.

[0062] For measuring the access flow rate, it is necessary to reversethe flow through the arterial and venous needles. One way of achievingthat is to reverse the needles manually.

[0063] Alternatively, FIG. 5 shows a valve 28 for performing the sameoperation. The arterial needle 5 is connected to an arterial inlet line29 of the valve and the venous needle 6 is connected to a venous inletline 30 of the valve. The blood pump is connected to a first outlet line31 of the valve and the returning blood from the dialyzer 11 isconnected to a second outlet line 32 of the valve.

[0064] The valve comprises a valve housing and a pivotable valve member33, which is pivotable from the normal position shown on the drawing toa reverse position pivoted 90° in relation to the normal position.

[0065] In the normal position shown in FIG. 5, the arterial needle 5 isconnected to the blood pump 8 and the venous needle 6 is connected tothe outlet of the dialyzer, via the drip chamber, see FIG. 2. In thereversed position, the arterial needle 5 is connected to the outlet ofthe dialyzer and the venous needle 6 is connected to the blood pump 8,as required.

[0066] An alternative design of the valve arrangement is shown in FIGS.7, 8 and 9. In the embodiment of FIG. 7, the arterial line 29 isconnected to an enlarged opening 29 a and the venous outlet line 30 isconnected to an enlarged opening 30 a, the openings being arranged inthe valve housing 28 a diametrically opposite to each other. Twoenlarged openings 31 a and 32 a are arranged in the valve housing 28 adiametrically opposite each other and displaced 90° in relation toenlarged openings 29 a and 30 a. The pivotable valve member 33 a isnormally arranged as shown in FIG. 7 and forms a partition dividing thevalve chamber in two semi-circular portions. The valve member has awidth, which is smaller than the peripheral dimension of the enlargedopenings. The valve member is pivotable 90° to a reverse position, shownin FIG. 9, in which the blood flows through the arterial and venousneedles are reversed.

[0067] During its movement from the normal to the reversed position, thevalve member 33 a passes through an idle position shown in FIG. 8, inwhich all four enlarged openings are interconnected, because the widthof the valve member is smaller than the peripheral dimension of theenlarged openings. By this idle position, harm to blood cells may beavoided. Such harm may be caused by high shear stresses, which may occurif the inlet line 31 to the blood pump or the outlet line 32 from thedialyzer is completely occluded. By means of the idle position, anotheradvantage is obtained, that the blood needles are not exposed to rapidchange of flows, which in some instances even may result in dislocationof the needles. When the valve member is moved from the normal positionto the idle position, the flow through the needles change from thenormal flow of, for example, 250 ml/min to essentially zero flow. Thevalve member may be placed in the idle position for some seconds. Then,the valve member is moved to the reversed position, and the flowsthrough the needles are changed from essentially zero flow to −250ml/min. In this way, a gentler switch between normal and reversed flowsmay be obtained.

[0068] It is noted, that the positions of the openings and the valvemember may be different so that the pivotal movement may be less than ormore than 90°. Moreover, the openings need not be arranged diametricallyin order to achieve the desired operation. Furthermore, the dimensionsof the enlarged openings in relation to the tubes and lines are not inscale, but the diameter of the enlarged openings is rather of the samedimension as the tube inner diameter, as appears more clearly below.

[0069] It is noted that the valve is constructed to have as few dead endportions as possible, in which the blood may stand still and coagulate.From the drawing, it is appreciated that no portion of the valve has adead end construction in any position of the valve body.

[0070] Furthermore, another schematic diagram incorporating a valve isshown in FIG. 10. FIG. 10 differs from FIG. 5 only in the placement ofthe pump 8 a, which in the embodiment according to FIG. 10 is placedbetween the arterial needle 5 and the valve 28. In this manner, thepressure across the valve body 33 is less compared to the embodimentaccording to FIG. 5. The operation is somewhat different. The blood pumpis stopped, and the valve is put in the reversed position. Finally, thepump is started and pumping the blood in the opposite direction byreversing the rotational direction of the pump.

[0071] In order to ascertain that no air is introduced into the patientin either position of the valve, it may be advantageous to add an airdetector 34 and 35 immediately before each of the arterial and venousneedle, or at least before the arterial needle. The air detectorstrigger an alarm should they measure air bubbles in the blood given backto the blood vessel. Normally, the air detector in the drip chamber issufficient for this purpose.

[0072] The detailed construction of a valve intended to be used in thepresent invention, is disclosed in FIGS. 12, 13 and 14. The valvecomprises a valve housing 36 comprising two inlet connectors and twooutlet connectors. All four connectors open into cylindrical valvechamber 41, the four openings being displaced 90° in relation to eachother.

[0073] As shown in FIG. 14, the valve comprises a blood inlet connector37 connected to the arterial needle 5 and a blood outlet connector 38connected to the venous needle 6. The connector portions are arranged asmale Luer connectors to be connected to flexible tubes ending with afemale Luer connector.

[0074] Furthermore, the valve comprises a circuit outlet connector 39connected to the blood pump 8 and a circuit inlet connector 40 connectedto the dialyzer outlet. The connector portions 39 and 40 are arranged asfemale Luer connectors to mate with male Luer connectors of the circuit.

[0075] As appears from FIG. 12, the cylindrical valve chamber 41 isclosed at the bottom. From the top, a valve member 42 may be introducedinto the cylindrical valve chamber. The valve member 42 comprises avalve partition 43 as appears from FIG. 13.

[0076] The valve member also comprises an operating wing 44, by means ofwhich the valve member may be pivoted 90° between a normal position, inwhich the valve partition 43 is situated as shown by dotted lines inFIG. 14, and a reversed position. The pivotal movement is limited by ashoulder 45 of the valve member 42, which cooperates with a groove 46 inthe valve housing. The shoulder 45 is provided with a protrusion 46 athat cooperates with two recesses 47 and 48 in the normal position andreverse position, respectively, to maintain the valve member in eitherposition. The groove 46 may be provided with a third recess (not shownin the drawing) in order to define said idle position. Such a thirdrecess is positioned in the middle between the two recesses 47 and 48.

[0077] The valve member and housing are provided with suitable sealingto ensure safe operation. The operation of the valve is evident from theabove description.

[0078] By studying the theoretical dialysate urea concentrationsresulting from a given dialyzer clearance K, a given access blood flowQa and a given blood urea concentration Cs in the systemic venous bloodreturning from the body, it is found that the effective urea clearanceKeff of the dialyzer, taking the cardiopulmonary recirculation intoaccount, is needed for the calculation of access flow. The effectiveclearance can be measured, for example as described in EP 658 352, thecontents of which is incorporated in the present application byreference.

[0079] Alternatively, the effective clearance can be calculated fromsimultaneous systemic venous blood Cs and dialysate Cd measurements ofurea concentrations, such as by blood samples.

[0080] The systemic blood urea concentration Cs may be measured by theso called stop flow—slow flow technique, where the blood flow issubstantially stopped for a couple of minutes to allow thecardiopulmonary recirculation to equalize. Thereafter, the pump is runslowly to fill the arterial line with fresh blood before taking theblood sample. The urea concentration in the so obtained blood sample isequal to the urea concentration Cs in the systemic venous bloodreturning from the body to the heart.

[0081] Alternatively to taking a blood sample, the dialysis fluid flowat the other side of the membrane is stopped and the slowly flowingblood is allowed to equalize with the dialysate at the other side of themembrane, whereupon the urea concentration of the dialysate is measuredto obtain the systemic venous blood urea concentration Cs.

[0082] A further method to obtain effective clearance is described in WO9929355. According to the invention described in WO 9929355, thesystemic blood concentration Cs is measured before or at the initiationof the treatment, for example by stop flow—slow flow technique withblood sample or equalization as described above. After obtaining validdialysate urea concentration values Cd from a urea monitor connected tothe dialyzer outlet line, the initial dialysate urea concentrationCdinit at the start of the treatment is extrapolated by the dialysateurea curve obtained. The content of WO 9929355 is incorporated herein byreference.

[0083] A still further method of obtaining systemic blood ureaconcentration Cs is to calculate the urea mass Mwh in the whole body andextrapolate the urea mass to the start of the treatment. By dividing thewhole body urea mass Mwh with the distribution volume V, the systemicblood urea concentration Cs at the start of the treatment is obtained.

[0084] By dividing the dialysate urea concentration Cd with the systemicblood urea concentration Cs and multiplying with the dialysate flow rateQd, the effective clearance Keff is obtained. It is advantageous tomeasure the effective clearance Keff at the initiation of the treatment.

[0085] Furthermore, in the method of the invention, the blood flows inthe arterial and venous needles are reversed. The dialysate ureaconcentrations in the two cases with normal position of the needles andwith reverse position of the needles may be calculated as follows, withreference to FIGS. 3 and 4.

[0086] The blood urea concentration Cs in the venous blood returningfrom the body is assumed unchanged when the lines are reversed, and thedialyzer clearance K is also assumed unchanged. For simplicityultrafiltration is assumed to be zero, but it is also possible to handlea nonzero UF.

[0087] The following notations are used:

[0088] Qco—Cardiac Output

[0089] Qa—Access flow

[0090] Qb—Blood flow in extracorporeal circuit

[0091] Qd—Dialysate flow

[0092] K—Dialyzer clearance

[0093] Keff—Effective dialyzer clearance

[0094] Cs—Blood urea concentration in systemic venous blood returningfrom the body

[0095] Ca—Blood urea concentration in the access

[0096] Cb—Blood urea concentration at the dialyzer inlet

[0097] Cd—Dialysate urea concentration

[0098] The definition of clearance is:

K=(removed urea)/Cb=Qd*Cd/Cb   (1)

[0099] Consider first the case in which Qa>Qb and the needles are in thenormal position. In this case Cb=Ca.

[0100] Removal from blood must equal appearance in the dialysate so that

K*Ca=Qd*Cd   (2)

[0101] A mass balance for urea at the point V, see FIG. 3, when mixingthe venous return blood with the blood from the access gives:

Ca*Qco=Cs*(Qco−Qa)+Ca*(Qa−K)   (3)

[0102] Thus, we obtain a relation between Ca and Cs.

[0103] By combining equations 2 and 3 we obtain:

Cd=(K/Qd)*Cs/[1+K/(Qco−Qa)]  (4)

[0104] The definition of effective clearance Keff implies that Cs shouldbe used in the denominator instead of Cb as normally used in dialyzerclearance, which means that

Keff=K*(Cb/Cs)=K/[1+K/(Qco−Qa)]  (5)

[0105] If we now turn to the case with reversed lines, see FIG. 4, westill have that what is removed from the blood must enter the dialysate,so that in this case

K*Cb=Qd*Cd   (6)

[0106] The flow in the fistula between the needles will be Qa+Qb and wecan calculate the blood urea concentration at the dialyzer inlet from aurea mass balance at the point P where the dialyzed blood enters theaccess again

Cb*(Qb−K)+Ca*Qa=Cb*(Qb+Qa)   (7)

[0107] We also have the mass balance at the point Q where the venousreturn blood meets the dialyzed blood in the access return flow:

Ca*Qco=Cs*(Qco−Qa)+Cb*Qa   (8)

[0108] By eliminating Ca and Cb we get

Cd=(K/Qd)*Cs/[1+(Qco/Qa)*K/(Qco−Qa)]  (9)

[0109] Since Cs, K and Qd in the two cases are unchanged, it is possibleto obtain the ratio of dialysate urea concentrations: $\begin{matrix}\begin{matrix}{{{Cd}\quad {{norm}/{Cd}}\quad {rev}} = {1 + {\left( {K/{Qa}} \right)/\left\lbrack {1 + {K/\left( {{Qco} - {Qa}} \right)}} \right\rbrack}}} \\{= {1 + {{Keff}/{Qa}}}}\end{matrix} & (10)\end{matrix}$

[0110] In practice, the two dialysate urea concentrations are probablybest found by a curve fit to the dialysate urea curves before and afterthe switch of lines, with an extrapolation to the time of switching fromthe respective side, see FIG. 6, which shows the urea concentration Cdof the dialysate during a normal hemodialysis treatment.

[0111] During a time period of about 10 minutes, marked with a ring inFIG. 6, the arterial and venous needles are reversed. After an initialtime period for allowing the urea monitor to measure accurately, theurea concentration with reversed lines is approximately 0.8 times theoriginal urea concentration, which means that Cdnorm/Cdrev=1.25. Thus,if Keff is 200 ml/min, as measured with the needles in the normalposition or estimated as described above, the access flow is 800 ml/min.

[0112] The effective clearance may also be obtained as a rough estimatefrom blood and dialyzer flows and dialyzer characteristics, e.g. fromthe dialyzer date sheet.

[0113] In the present specification, there are used three differentclearances, namely dialyzer clearance, effective clearance and wholebody clearance. If dialyzer clearance is 250 ml/min for a certain bloodflow rate and dialysate flow rate, the effective clearance is normally 5to 10% lower, such as 230 ml/min. The whole body clearance is still 5 to15% lower, such as 200 ml/min. The dialyzer clearance is the clearanceas measured directly on the dialyzer. The effective clearance is theclearance also taking into account the cardiopulmonary recirculation.Finally, the whole body clearance is the effective clearance furthertaking into account other membranes in the body restricting the flow ofurea from any part of the body to the dialysate. The concept of wholebody clearance is described in WO 9855166, the content of which isherewith incorporated by reference.

[0114] The effective clearance used in the formula may also be obtainedfrom a measurement according to the method described in EP 658 352mentioned above, with the needles in the normal position. This will givea measure of the effective plasma water urea clearance, which then hasto be converted to whole blood clearance. The method of EP 658 352essentially comprises that the conductivity of the dialysis fluidupstream of the dialyzer is increased by for example 10% and thenreturned to the original value. The result at the outlet side of thedialyzer is measured and results in a measure of the effective clearanceKeff of the dialyzer.

[0115] Alternatively, the effective clearance may be calculatedaccording to equation Keff=Qd*Cd/Cs. The systemic venous ureaconcentration may be measured at the same time as the dialysate ureaconcentration Cd, or by the methods described above.

[0116] Another method would be to use the value of total body urea massMurea obtained by the method according to WO 9855166, mentioned above.By obtaining the urea distribution volume V by Watson's formula or anyother method, the venous urea concentration would be approximately:

Cs=Murea/V   (11)

[0117] In the method of WO 9855166, the relative whole body efficiencyof the dialyzing process Kwb/V is obtained. Note, that whole bodyclearance is used, as indicated by the subscript wb. According to saidWO 9855166, urea concentration is proportional to the relative wholebody efficiency according to the formula:

Kwb/V=(Qd*Cd)/m   (12)

[0118] Thus, if (Kwb/V) is used instead of Cd in the above equation(10), a similar result is obtained, if it is presumed that m isconstant, i.e. the measurement must be extrapolated to the same timeinstance:

(Kwb/V)norm/(Kwb/V)rev=1+Keff/Qa   (13)

[0119] As is mentioned in said WO 9855166, it is possible to calculatethe relative whole body efficiency only from dialysate urea measurement.Since we are interested only in the ratio in the normal and reversedposition, we do not need to calculate the actual Kwh.

[0120]FIG. 11 shows a plot of the relative whole body efficiency K/V(min⁻¹). The period with reversed lines is shown inside a circle. In allother respects, the same discussion applies as is given above.

[0121] The calculations above assume that the extracorporeal blood flowrate Qb does not exceed the access flow rate Qa. If this is the casethere will be access recirculation and the flow in the access will bereversed when the needles are in the normal position. The calculation ofdialysate urea concentration is unchanged for the needles in reversedposition, but has to be modified for the needles in normal position.Calculations corresponding to those above show that the ratio abovebetween dialysate urea concentrations for normal and reversed needlepositions will be:

Cd norm/Cd rev=1+Keff/Qb   (14)

[0122] where Keff is the effective clearance with the effect ofrecirculation included, that is with the needles in the normal position.

[0123] The only difference is that the calculation will now give theextracorporeal blood flow Qb instead of the access flow. This blood flowis known, so in practice this means that when the result is an accessflow rate Qa close to the blood flow rate Qb, recirculation should besuspected, and this always means that the access has to be improved.

[0124] Keff/Qb is a figure lower than one, normally for example 0.6-0.9.Keff/Qa should be considerably lower, for example 0.1-0.4. Thus, when Cdnorm/Cd rev approaches or is lower than a predetermined number, such as1.2 or 1.5, further calculations should be done for determining ifaccess recirculation is present.

[0125] A simple procedure is to decrease the blood flow Qb somewhat. Ifthe dialysate concentration then decreases, this means that there is noaccess or fistula recirculation at least at the lower blood flow.

[0126] The above calculations can also be made for the situation whereultrafiltration is present. However, it is a simple measure to reducethe ultrafiltration to zero during the measurement interval. Moreover,the error induced by ultrafiltration is small and may be neglected.

[0127] The measurement should be performed during a time interval, whichis considerably larger than 30 seconds so that cardio-pulmonaryrecirculation has been developed. The measurement time for obtainingvalid results may be 5 minutes with the needles reversed, whilemeasurements with the needles in correct position may be done in 5minutes or continuously during the treatment.

[0128] The method is also applicable to the methods of treatmentcomprising infusion of a dialysis solution to the blood before or afterthe dialyzer, called hemofiltration and hemodiafiltration. The result isthe same as given above.

[0129] If the access is a venous catheter, there is no cardio-pulmonaryrecirculation and the calculation becomes simpler. The result is thesame, except that the effective clearance Keff is replaced by thedialyzer clearance K, since the systemic venous urea concentration Csbecomes the same as the dialyzer inlet urea concentration Cb.

[0130] It should be noted that all flow rates, clearances and ureaconcentrations in the calculations relate to whole blood. Approximately93% of plasma is water, depending on the protein concentration, andabout 72% of erythrocytes is water. Depending on the hematocrit value,the blood water volume is 10-13 % lower than the volume of whole blood,see for example Handbook of Dialysis, Second Edition, John T. Daugirdasand Todd. S Ing, 1994, page 18.

[0131] The effective urea clearance obtained according to EP 658 352relates to blood water, and must therefore be increased by 10-13 %before being used in the present formulas. Blood urea concentrationvalues obtained from a laboratory relate in general to plasma, and musttherefore be decreased by about 7% in order to relate to whole blood.

[0132] Alternatively, all urea concentrations, flow rates and clearancesmay be used as relating to blood water. The effective clearance is thenused unchanged, but the calculated access flow will relate to bloodwater, and has to be increased by 10-13 % to relate to whole blood.

[0133] The invention has been described above with reference to use inthe human body and using urea as a marker for measuring access flow.However, any other substance present in blood and which can be measuredat the dialysate side of the dialyzer may be used according to theinvention, such as creatinine, vitamin B12, beta-two-microglobuline,NaCl or any combination of ions. Another alternative is to measureconductivity.

[0134] It is also possible to measure a property proportional to theconcentration, since it is the ratio that is involved in the equations.Thus, urea concentration may be measured by measuring conductivitydifferences after passing the urea containing fluid through a ureasecolumn, and such conductivity difference can be used directly in placeof the concentration values in the equations.

[0135] Other indirect methods of measuring any of the above-mentionedsubstances concentrations may be used as long as the measurements aremade at the dialysate side of the dialyzer. Another alternative is tomeasure the blood urea concentrations by any known method, either beforeor after the dialyzer, since these concentrations are proportional tothe concentrations in the formulas.

[0136] The invention has been described above with reference to use inthe human body. However, the invention can be used in any tube systemwhere a fluid is passed and a portion thereof is taken out for dialysis,such as in beer or wine production.

1. A switch valve for use in an extracorporeal blood flow circuitcomprising: a valve housing having a chamber, four openingscommunicating with the chamber; and a valve member located in the valvechamber and movable therein to change the direction of blood flow amongthe openings, wherein the width of the valve member is smaller than aperipheral dimension of the openings.
 2. A switch valve for use in anextracorporeal blood flow circuit comprising: a valve housing having achamber, four openings communicating with the chamber; and a valvemember located in the valve chamber and movable therein to change thedirection of blood flow among the openings, wherein the valve member isarranged to adopt an idle position in which all four openings areinterconnected.
 3. A switch valve for use in an extracorporeal bloodflow circuit comprising: a valve housing having a chamber, four openingscommunicating with the chamber; and a valve member located in the valvechamber and movable therein to change the direction of blood flow amongthe openings, wherein movement of the valve member does not ever fullyblock any one of the openings.
 4. The switch valve claimed in claim 1 or3, wherein the valve member is arranged to adopt an idle position inwhich all four openings are interconnected.
 5. The switch valve claimedin claims 1 or 2, wherein the valve member does not ever fully block anopening.
 6. The switch valve claimed in anyone of claims 1 or 2 or 3,wherein the valve member is pivotable in the chamber.
 7. The switchvalve claimed in claim 6, wherein the valve member is pivotable from anormal blood flow position, through an idle position and to a reverseblood flow position.
 8. The switch valve claimed in claim 6, wherein thevalve member is pivotable through 90°.
 9. The switch valve claimed inanyone of claims 1 or 2 or 3, wherein the openings are disposed on thevalve housing diametrically opposite to each other.
 10. The switch valveclaimed in claim 9, wherein the valve chamber is cylindrical and theopenings are spaced 90° relative to each other around the chamber. 11.The switch valve claimed in anyone of claims 1 or 2 or 3, wherein theopenings are each provided with a respective connector.
 12. The switchvalve claimed in anyone of claims 1 or 2 or 3, wherein the valve memberforms a partition dividing the valve chamber in two portions.
 13. Theswitch valve claimed in claim 12, wherein each of said portions issemi-circular.
 14. The switch valve claimed in anyone of claims 1 or 2or 3, wherein the valve member includes a valve partition that extendsinto the valve chamber and a wing with which the valve member can bemanually moved.
 15. The switch valve as claimed in claim 14, wherein thevalve member includes a shoulder, which limits the pivoting movement ofthe valve member in the chamber.
 16. The switch valve as claimed inclaim 15, wherein the shoulder cooperates with a groove on the peripheryof the valve chamber.
 17. The switch valve as claimed in claim 16,wherein the groove has recesses defining normal and reverse positions.18. The switch valve as claimed in claim 17, wherein the groove also hasa recess defining the idle position.
 19. The switch valve as claimed inclaim 11, wherein a first and a second of said connectors extenddiametrically opposite from the valve housing and a third and a fourthsaid connectors are symmetrically inclined by less then 90 degrees withrespect to a direction of the first connector.
 20. The switch valve asclaimed in claim 2 or in claim 3, wherein the peripheral width of thevalve member is smaller than a peripheral dimension of the openings. 21.An extracorporeal circuit comprising: a dialyzer blood compartment; aswitch valve according to anyone of claims 1 or 2 or 3, wherein thevalve comprises a connector for each of said openings, said connectorsincluding: a blood inlet connector, a blood outlet connector, a circuitinlet connector, and a circuit outlet connector, and wherein the circuitoutlet connector is connected to an inlet of the dialyzer bloodcompartment and the circuit inlet connector is connected to an outlet ofthe dialyzer blood compartment.
 22. An extracorporeal circuit accordingto claim 21 comprising a line connecting the circuit outlet connector tothe inlet of the dialyzer blood compartment, a line connecting thecircuit inlet connector to the outlet of the dialyzer blood compartmentand two further lines connecting the remaining connectors to venous andarterial needles.
 23. An extracorporeal circuit according to claim 21,wherein a peripheral width of the valve member is smaller than aperipheral dimension of the openings.
 24. A blood treatment equipmentcomprising: a dialyzer having a blood compartment and a dialysis fluidcompartment separated by a semi-permeable membrane, an arterial needlefor connection to a patient's fistula, a venous needle for connection toa patient's fistula downstream with respect to the arterial needleconnection, a switch valve comprising: a valve housing having a chamber,four openings communicating with the chamber, a valve member located inthe valve chamber and movable therein to change the direction of bloodflow among the openings, wherein movement of the valve member does notever fully block anyone of the openings, and a connector for eachrespective opening, said connectors including: a blood inlet connector,a blood outlet connector, a circuit inlet connector connected to anoutlet of the dialyzer blood compartment, a circuit outlet connectorconnected to an inlet of the dialyzer blood compartment, an arterialline connecting the arterial needle to the blood inlet connector, avenous line connecting the venous needle to the blood outlet connector,the switch valve being operable between a normal position, wherein fluidis directed from the blood inlet connector to the circuit outletconnector and from the circuit inlet connector to the blood outletconnector, and a reversed position, wherein fluid is directed from theblood outlet connector to the circuit outlet connector and from thecircuit inlet connector to the blood inlet connector.
 25. A bloodtreatment equipment according to claim 26, comprising a blood pumpoperating on a line segment extending between said circuit outletconnector and said inlet of the dialyzer blood compartment.
 26. A bloodtreatment equipment according to claim 26, comprising a blood pumpoperating on said arterial line.
 27. A blood treatment equipmentaccording to claim 26, wherein a peripheral width of the valve member issmaller than a peripheral dimension of the openings.
 28. A bloodtreatment equipment according to claim 26, wherein the valve member isarranged to adopt an idle position in which all four openings areinterconnected.